Huh, I think it depends on if you have a kettle large enough for a full wort boil. If yes - then go with BIAB, and you should be fine in terms of a 55%+ brew house efficiency. You can always add DME to boost the gravity.
If you have a smaller kettle and are going to top off the fermentor with water, then I think that is partial mash (with a large grain bag).